Don't want people to sit in your cafe with their laptops? There's a simple solution: don't have WiFi. But if you're going to have WiFi then for fuck's sake have fucking WiFi. And if your WiFi isn't working, if it's down and it's gonna be down all day, you might wanna mention that to people before they wait in line, buy a coffee, leave a tip, sit down, and pull out their computers. Because then each and every one of those computer users is going to walk up to the counter and ask if you have WiFi. It's an asshole move to look at each laptop computer user/customer in turn like they've just asked you if you have herpes. And if it really kills you to sneer out, "Yeah, we have WiFi, but it's down," then put a little sign on the door that says the WiFi's out. Then laptop users won't bother you with their questions, their presence, or their patronage.
UPDATE: And laptop users? Tip based on the amount of time you intend to spend in the cafe, not on the price your beverage; buy your refills; share tables; and always remember that you're not actually in your office.